Sound-generator



M. l. PUPIN.

SOUND GENERATOR,

APPLICATION FILED FEB. 4, 191a.

Patented Dec. 13, 1921.

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Specification of Letters Patent. Patented Dec. 13, 1921.

Application filed February4, 1918. Serial No; 215,291.

Connecticut, have invented certainnew and-- useful Improvements in Sound-Generators; and I do hereby declare the following to be a full, clear, and exact description of the invention, such as will enable others skilled in the art to which it] appertains to make and use the same. g

This invention provides a new form of oscillator for generating compressional waves, the object being to produce a rugged durable device ,of such character as toafford a large radiating surface as well as a large surface for the action of the impressed moving force, to permit the generation of sound waves of large energy at all frequencies. For this purpose I employ as the oscillating sound producing source an electrical condenser consisting of a cylindrical core constituting one element of the ,condenser, and a surrounding cylindrical shell,

separated by a dielectric from the cylindrical core, and constituting the other element of the condenser. When variations of electrical potential are applied to such acondenser, the cylindrical shell will be set into vibration at a frequency corresponding to the frequency of the electrical variations, and will produce sound waves of corresponding frequency. In this way large amounts of electrical energy may be transformed efi'iciently into compressionalwaves of any desired frequency. It will be noted that the apparatus has the advantage that the production of notes of high pitch does not involve any reduction of the radiating power of the apparatus, as is the case in sound producers such as piano strings and thelike.

. In practice it is advisable to make use of electrical resonance to apply the electrical energy to'the actuation of the oscillator in the form of potential variations of great amplitude; For the purpose of further increasing the elliciency of transformation of the electrical energy into mechanical energy, it is desirable, in some cases, so to choose or design the cylindrical shell that it has a naturalperiod of mechanical vibration of the frequency of the sound waves to be produced. If electrical resonance is used in connection with a cylinder so chosen or de signed, the resonant circuit will, have, the

same frequency, as will be understood.

A moreparticular. understanding of the natureof the invention may be had from the following description of the preferred form oscillator, together with means for transforming electrical energy into mechanical energy to actuate the oscillator.

In these drawings, Figure '1 is a crosssection' of the oscillator used for sending pl rposes; Fig. 4 represents, somewhat diagrammatically, the position of the oscillator within a cylindrical parabolic mirror, the

focal line of which is the axis oftheoscillator, and Figs. 2 and 3 are diagrams "representing the connections for applying electrical energy in two ways to the actuationsv of the oscillator.

Fig. 1 illustrates an oscillator which I have constructed as an embodiment of my nvention, and in describing it I shall refer 75 to the dimensions which have actually been employed. This particularity of description is for the purpose of enabling those skilled in the art, by the consideration of one particular form in which the invention has been embodied, to understand the nature'and principles of'the invention, and it is not to be taken as limiting in any waythe scope of "the invention as defined in .the accompanying claims.

In this'device, 2 is a steel tube'about .15 centimeters thick and having an internal diameter of 2 centimeters. 3 is a steel cylinder 1.975 centimeters in diameter and concentrically mounted within tube 2 so that the cylindrical air space between them is .0125 centimeters thick. The cylinder 3'is held in position, and is insulated from'tube 2, by the insulating fiplugs 4. In order to increase largely the d1 erence of electrical potential which may be maintained between theyjcylinder and shell I fill the space between tliem with compressed air. In this way, provided the insulating plugs 4 are suitably constructed to limit the electrical leakage from the cylinder. to the shell, as indicated, the electrical potential gradients can easily be made large, about 5- 10 'volts per centimeter, that is, an electrical otential may be maintained between the c inder and she-ll of about 12,500 volts. t will be observed that the device constitutes a cylindrical electrical condenser.

Insulated-steel caps 5- and 6 serve to strengthen the structure so that it will withstand high aircompressions in-Jlw volts is impressed between shell 2 and cylinder .3- the mechanical force F acting between them will have the following value in dynes per square centimeter:

1 E r 1 l ss) 9X10 Pt where h is the thickness of the air shell which, as indicated above, is .025 centimeters. 1

This expresion for F may also be written:

That is to say, the mechanical force acting:

between the cylinder and shell is equal to a constant force plus an alternating force of double the frequency of the impressed elec- 'trical difference of lpotential; that is, the alternating mechanical force rises from zero "to a maximum and falls again to zero for each'half wave of the alternating electromotive force.

\ mately.

'. vThe alternating mechanical force acting I upon the whole surface of a tube cent1- meters long will be 10,365 grams.

In consequence of the action of this alternatingfforce the steel shell will vibrate mechanically with the frequency speed 2 p. A similar result can be obtained by charging the oscillator to a constant 'electromotive force of volts and then superposing upon it an "alternating electromotive force equal E Y to E c052 pt.

The amplitude of these vibrations may be increased by causing the frequency of the force and the natural frequency of mechanical vibration of the tube to approach each other and will be a maximum when resonanc'e is reached. Sincethe amount of energy in the form of sound waves radiated from the tube is proportional to the square of the amplitude of vibration it is clear that by resonance the sound energy which is emitted by radiation in the surrounding me- .dium is increased.

A convenient method of actuating electrically the cylindrical oscillator of the character described is represented in two forms in Figs. 2 and 3. In Fi 2 the core 3 and the shell 2 of the cylin rical oscillator are connected electrically by a circuit including inductance 11 and a secondary 12 of a transformer whose primary '13 receives an im- 1,a9e,a77--- pressed hig from a suitable source, which may be, for instance, a vacuum tube osclllator of the large electrical output, say several hundred watts. The inductance 11 is variable so' that the circuit may be tuned to increase the difference of electrical potential in the oscillator to any desirable value byelectrical resonance.

The arrangement of Fig. 3 differs from that of Fig. 2 in that an auxiliary condenser 14 is connected in parallel with the oscillator and the two'are kept charged b a direct current source 15 to a suitable di erence of potential, preferably equal to the amplitude of the. alternatin voltage and adjustable by means of the adiustableresistance 16. In this mannerthe difference of alternating potential. to be su plied by the alternating source through tlie transformer 12, 13, can be reduced to half of the value necessaryan the arrangement of Fig. 2, but its f uency must be doubled. Under certain con itions the arrangement of Fig. 3 would offer substantial advanta es.

' In a steel shell of the character and dimensions above described the natural frequency of mechanical vibrations 1s given by,

the formula, taken from mathematical treatises on sound:.

h frequency electromotive force well known form capable of delivering. a

It is obvious from this formula that the circumference of the tube is equal to one wave length of the natural vibration of the tube. For example, if v is equal to the usual value for tool steel, that is 5X10 c.m. per

, sec. then in mycase 21' being equal to 2 centimeters the natural frequency of vibration will be approximately 8X10 pps.

This value of the natural frequenc will be diminished by the resistance reaction of the surrounding medium. If the surround mg medium is air, the diminution will be inappreciable. If it is water, the diminution is considerably increased, but it can be kept within' reasonable limits by increasin the thickness of the shell. Thus, when the t ickness of the shell is .15 centimeters the reduction in the frequency given above will be less than 1 per cent.

When the steel shell is vibrating in water under the action of a 'ven'force, the am litude of vibration can shown to be, wit in wide limits, independent of the thickness when resonance is established. The thickne'ss of the steel shell, on the other band,

should be as large as practicable, because the greater its thickness the greater can be the therefore a radiation 11 times as large be- V is that thecylindrical sound axis on the focal line of a bolic'mirror, as illustrated in Fig. 4. In-

cause the radiation is proportional to the square of the amplitude of vibrations.

When the oscillator is under water the damping is very considerable and no ver sharp resonance exists, so that an oscillator of natural frequency equal to 8X10 pps can be operated by a force of frequency 10 pps without appreciable loss in radiation. This small loss will be more than compensated by the advantages gainedfrom the higher frequency.

Another advantage which the steel tube offers, particularly when its frequency is very high, say from 5X10 to 15x10? pps waves which it produces in the surrounding medium can be easily transformed into plane waves which will be confined to a small space and form 'a parallel beam of sound waves.

This is done by placing the tube oscillator with its cylindrical para:

this figure 17 is a cylindrical'parabolic mirror the focal axis of Whichcoincides with the axis of the cylindrical oscillator 2, 3. According to'well known rules the sound waves radiated from the oscillator will be reflected by the cylindrical parabolic mirror to form a plane wave or beam of compressional waves sound confined to the prism parallel to the axis of the mirror and the cross-section of which is equal to the opening of the parabolic mirror, provided the wave length is sufficiently small in comparison with the linear dimensions of the mirror. This condition is easily fulfilled by frequencies of 8X10 pps or higher frequencies. In this manner a parallel beam of compressional waves sound is formed the direction-of which can changing the position of the mirror or otherthe elastic displacements are easily be changed by wise, just as in the ordinary operations. p It can be shown theoretically and roved Searchlight experimentally that the rate of ra iationfrom an oscillator excited in the manner indicated will be about one watt when it is placed in water. This is 'due to the fact that excessively small, somewhat "less than 10 centimeters. Without resonance they would be considerably smaller.

Ina companion application, Serial No.

opening of the vice above described is particularly adapted to the production of compressional waves of y a frequency far above audibility for use in a system of signaling involving a directed tram of compressional waves together with a suitable receiver capable of receiving and rendering audible such waves, and that the principles of electrical and mechanical resonance are made that device. It will, however, be understood that the invention is applicable, and of great value, in connection with the production of audible and musical sounds and that the oscillator may be made of any suitable stifi material and may or may not be resonant to a particular frequency of audible sounds. F urthermore, by controlling the electrical variationtelephonically or otherwise in ac-- cordance with speech or musical sounds, the device, or a series of them, may be used as an effective producer or reproducer of such sounds.

I claim: A

1. A generator of compressional waves comprising a stiff cylindrical body constituting one element of an electrical condenser, a second condenser element associated therewith, a dielectric interposed between said elements for preventing a conductive flow of current therebetween, and means for imparting to the condenser varying electromotive forces of a frequency corresponding to the frequency of the compressional waves to be roduced.

2. generator of compressional waves comprising a stiff cylindrical body constitutlng one element of an electrical condenser,

a second condenser element associated therewith, a dielectric interposed between said two condenser elements having a natural.

frequency substantially the same as the frequency of the compressional waves to .be produced.

3. 'A generator of compressional waves use of in connection with between, and means for impartin comprisin an electrical condenser having a cylindrica core and a cylindrical shell surrounding the core, a ielectric interposed between the shell and the core for preventing a conductive flow of current thereto the condenser. varying electromotive orces of a frequency corresponding to the frequency of the compressional waves to be produced.

4. A generator of compressional waves comprising a stiff cylindrical body constituting one element of an electrical condenser, a second condenser element electrostatically associatedtherewith, a dielectric inter-. posed between said elements for preventing a conductive flow of current therebetween, and means for imparting to the condenser varying electromotive forces of a frequency substantially corresponding to the natural period of mechanical vibration of the'said body, whereby the electrical energy causes the body to oscillate resonantly. 5. A generator of compressional waves comprising a stiff columnar body constituting one element of an electrical condenser, a second condenser element electrostatically associated therewith and a dielectric interposed between said elements for preventing a conductive flow of electric current between said elements when an electromotive force is v applied to the condenser.'

6. A generator of compressional waves, comprising a stiff columnar body constituting one element of an electrical condenser, a second condenser element electrostatically associated therewith, a dielectric interposed I between said elements for reventing a conductive flow of current therebetween, and means for imparting to the condenser var ing electromotive forces of a frequency su period of mechanical oscillation of the said bQCl-y. i

7. A generator of compressional waves comprisin a stiff columnar body constituting one e ement of an electrical condenser, a second condenser element electro statically associated therewith, a dielectric interposed between said elements for reventing a conductive flow of current therebetween, and means for imparting to the condenser vary-v corresponding to the frequency of the compressional waves to be produced.

8. A generator of compressional waves comprising an electrical condenser having a columnar core and a concentric columnar shell of like configuration surrounding the core and separated therefrom by a dielectric of uniform thickness throughout for.

preventing a conductive flow of electric (airrent therebetween, and means for imparting ing electromotive forces having a frequency to the condenser varying electromotive forces of a frequency corresponding to the frequemg of the compressional waves to be produce 9. A generator of compressional waves com rising an electrical condenser having a cy indrical core and a concentric cylindrical shell surrounding the core, adielectric interposed between the shell and the 'core for 'preventih a conductive flow of electric current there etween, and means for imparting to the condenser varying electromotive forces of a frequency corresponding to the frequency of thecompressional'waves to be produced.

10. A sound enerator consisting of a cylindrical electrical condenser, one element of which is a stiff body and in which the dielectric separatin the two condenser elements'is compresse gas, and means for impartin to the condenser potential variations of a re' uency corresponding to the frequency o thesou'nds to be produced.

11. A sound enerator consisting of a stifi cylindrical bOC? constituting one element of an electrical condenser, a second condenser element electrostatically associated therewith, and means for imparting to the condenser varying electromotive forces of a frequency substantially corresponding to the natural period of mechanical oscillation of the said body, said means including a source of electrical energy and a resonant electrical circuit connected to the saidsource and to the two condenser elements and having a natural frequency substantiall e ual'to the said frequencyof mechanica vi ration of the oscillator body.

In testimony whereof I afiix my si ature'.

' M. I. PIN. 

